By graphic adventure, I mean point & click LucasArts-type games.
Those games have a mostly linear structure in nature, and usually don't offer as many variants as other games types like action, rpg, strategy, which makes this genre difficult to implement a multi-player feature.
I'd like to know if there has been any attempts on doing such a thing, and if it would be viable, as players going offline or leaving a game in the middle would affect significantly the other players' game.

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All I know of is sarien.net which has a 'multiplayer' option, which is really just that you can see and chat with other people who are in the game (and in the same adventure room)
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thedaianJun 20 '11 at 21:13

4 Answers
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Of course I can't say for sure since I haven't played all the games that there are in the world, but I seriously doubt that such a thing exists. I even doubt that it is viable. Here's why:

A large part of the fun in these adventure games comes from figuring things out from beginning to end. If another player partially does that for you (because you just joined the game, or because the game requires you to only solve a part of the puzzle) it kind of takes away the fun.

Adventure games are usually lengthy affairs, where you spend a lot of time in single chapters of the game. Having multiple players play this kind of game together would require them to play together for a very long time, which rarely works out even if you have buddies who want to play with you.

And last but not least: Adventure games are story-driven by design. I find it hard to imagine that these kinds of games would have a lot of replay value.

But of course, if you have some awesome idea to actually put this into practice, everyone's welcoming you. The gaming market is certainly not oversaturated with original ideas.

Whether a second player takes away the fun depends on the nature of the puzzle and of both players. Consider Portal 2 Co-op, in turn based on Valve's observation that many players of Portal 1 enjoyed solving its puzzles with a partner. If there hasn't been a multiplayer G.A. before, the possibility of good multiplayer-oriented G.A. puzzles hasn't had the opportunity to strut its stuff.
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doppelgreenerJun 20 '11 at 21:37

Graphic adventure games of the type you describe were never made multiplayer for one main reason: By the time computer networking was ubiquitous, the graphic adventure game genre was already dead. The few graphic adventures still being made were being made as niche games for a niche audience, they intentionally weren't breaking new design ground.

Yes, there are design challenges for making adventures work in a multiplayer context. But there are always design challenges no matter what you make; design challenges can be overcome. The reason there are no multiplayer adventure games is that the market had decided it didn't want adventure games long before multiplayer ever became a real option.

In SELEAG,we created a multiplayer engine for SCUMM based flash games: TIMEMESH. The idea was to create two teams of players which have to solve the same story but with slight variations.
There are cross-bars in the adventure game. A cross-bar is a shared blocking between two teams playing a match. Until both teams have gone through 2 parallel challenges neither of them can progress in the adventure game.

They have a chat enabled to collaborate in solving this blocking points (which sometimes were minigames embedded in the main game).